As a method for producing a carbon fiber, there is known that a method for growing a carbon fiber using a catalyst as a core, namely, a so-called chemical vapor deposition method (hereinafter referred to as CVD method). As the CVD method, a method in which a catalytic metal is supported on a carrier for use and a method in which an organic metal complex is thermally decomposed in a vapor phase so as to generate a catalyst without using a carrier (fluidized vapor phase method) are known.
As the carbon fiber obtained by the method of generating a catalyst in a vapor phase (fluidized vapor phase method), PTL 5 shows a carbon fiber having the total metal element content of 0.3 to 0.7% by mass and the transition metal element content of 0.1 to 0.2% obtained by a method (fluidized vapor phase method) in which an organic metal complex such as ferrocene and a carbon source such as benzene are fluidized, and the carbon source is thermally decomposed under a hydrogen atmosphere using a metal particle as a catalyst obtained by thermal decomposition of the metal complex. The carbon fiber obtained by this fluidized vapor phase method has many defects in a graphite layer and has a problem that without heat treatment at a high temperature, electric conductivity does not emerge even if being added to a resin or the like as filler. Thus, with the fluidized vapor phase method, it is difficult to inexpensively produce a carbon fiber having desired properties.
On the other hand, a method using a catalyst carrier is roughly divided into (1) a method using a platy substrate carrier; and (2) a method of using a particulate carrier. With the method (1) using a platy substrate carrier, since the size of the catalytic metal to be supported can be arbitrarily controlled by applying various film formation techniques, this method is usually used in laboratory demonstration of research. For example, NPL 1 discloses that using those in which an aluminum layer having thickness of 10 nm, an iron layer having thickness of 1 nm, and a molybdenum layer having thickness of 0.2 nm are generated on a silicon substrate can give a tube-like multiwall nanotube or a double-wall nanotube having a fiber diameter of approximately 10 to 20 nm. Also, PTL 4 discloses a catalyst by supporting a metal composed of a combination of Ni, Cr, Mo and Fe or a combination of Co, Cu, Fe and Al on a platy substrate carrier by a sputtering method or the like. And PTL 4 describes manufacture of a carbon fiber therewith. In order to use the carbon nanotube as filler obtained by this method using a platy substrate carrier to be added into a resin or the like, it is necessary to be separated from the substrate and collected. The carbon nanotube collected as the above substantially contains only catalytic metal component as impurities, but since generation efficiency of the carbon nanotube with respect to a catalyst mass is markedly low, the content of the catalytic metal component in the carbon nanotube is likely to be high. Moreover, if this method is to be industrially utilized, since a platy substrate surface area can not be ensured unless a number of substrates are arranged, not only that device efficiency is low but also many processes such as supporting of the catalytic metal on the substrate, synthesis of the carbon nanotube, collection of the carbon nanotube from the substrate and the like are needed, which is not economical, and industrial utilization has not been realized yet.
On the other hand, with the method (2) using the particulate cancer, since a specific surface area of the catalyst carrier is larger than that of the method using the substrate carrier, not only that the device efficiency is favorable but also a reactor used for various chemical synthesis can be applied, and this method has merits that realizes not only a production method based on batch processing such as the substrate method but also continuous reactions.
However, with the method using the particulate carrier, a catalyst carrier is unavoidably mixed in a carbon fiber product, and it is difficult to obtain a carbon fiber with high purity.
As a method for reducing the amount of impurities in the carbon fiber obtained by the method using the particulate carrier, (1) a method of heat treatment at a high temperature; and (2) a method of washing and removing with acid or base are known, but both of the methods have complicated processes and are not economical. Particularly, in the washing and removing of the impurities with acid or base, since the catalyst carrier and the catalytic metal in the carbon fiber are covered by a carbon overcoat in many cases, it is difficult to fully remove the impurities unless the carbon overcoat is removed by using an oxidizing acid such as nitric acid or by performing partial oxidization. If an oxidizing acid is used, not only the carbon overcoat on the surface of the carrier or the catalyst but also the carbon fiber itself might be damaged and become defective. The carbon fiber affected by an acid might have lowered electric conductivity or lowered heat conductivity, or dispersibility or filling performance into a resin or the like might be deteriorated.
Various catalysts for manufacturing a carbon fiber are proposed. For example, PTL 1 discloses a catalyst containing Fe element and at least one element selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, Tc, and Re. Specifically, the PTL 1 discloses that the catalyst is obtained by supporting a metal element composed of a combination of Fe and Mo, Fe and Cr, Fe and Ce, Fe and Mn or the like on a carrier using an impregnating method.
PTL 2 discloses a catalyst obtained by coprecipitation of a metal having fibril-forming catalytic properties composed of Fe or a combination of Fe and Mo and a carrier metal component such as Al, Mg and the like. It is disclosed that using this catalyst, a carbon fiber having the content of impurities from the catalytic metal of 1.1% by mass or less and the content of the impurities from the catalyst carrier of 5% by mass or less can be obtained.
PTL 3 discloses a catalyst obtained by coprecipitation of a catalytic metal component composed of a combination of Mn, Co, and Mo or a combination of Mn and Co and a carrier metal component such as Al, Mg and the like.